Abstract: A portable alphanumeric and symbol drafting device which essentially comprises a mechanically functioning keyboard unit, which is flexibly connected to an electronics package unit. Accordingly, the keyboard unit, which preferably weighs on the order of only two pounds, comprises a singularly useful device for a draftsman, since it allows a string of characters to be most accurately applied upon a drawing surface, by manipulation of only the keyboard unit. The keyboard unit may also include an attachment for mating with a standard parallel-arm drawing device. The keyboard unit comprises a first longitudinal edge, which is used to establish an origin or base line for the string of characters to be lettered and includes a carriage means for supporting a drafting pen in a manner to allow it to be operably controlled for a longitudinal (x-axis) and transverse (y-axis) movement proximate this first longitudinal edge of the housing.

Abstract: A two-stage commutation circuit for an inverter, each having a different energy storage capability, commutates the main semiconductor switches by creating a commutation pulse suitable for the different levels of load current. This improves overall inverter efficiency by decreasing the circulating commutation current at light loads due to reduced size of the commutation and makeup pulses utilized at the lower load current levels. The control circuit for the dual commutation circuits includes a voltage detector for sensing the level of the DC input voltage and current detectors for sensing the times at which the load current is essentially zero so that the transistion between the commutation circuits will occur without perturbations in the cyclic operation of the inverter.